1. Field of the Invention
The present invention relates to the field of electrical technology/electronics and concerns a surface acoustic wave component. The component according to the invention is useful in filters, delay lines, resonators, and oscillators, but also sensors and identification components, to the extent that their method of functioning is based on surface acoustic waves.
2. The Prior Art
Surface acoustic wave components are known, in which at least two transducers having natural unidirectionality are disposed on a piezoelectric crystal substrate. In this connection, the transducers have an interdigital electrode structure having prongs and bus bars. At least two of the prongs form a transducer cell that has at least one excitation center for exciting an electrical potential wave and at least one reflection center for reflection of electrical potential waves.
On piezoelectric crystal substrates, surface acoustic waves contain not only mechanical particle displacements but also an electrical potential that spreads at the same velocity and wavelength as the mechanical particle displacements. This electrical potential, as an integral part of the surface acoustic wave, is referred to as an electrical potential wave.
A prerequisite for the existence of unidirectional properties of interdigital transducer cells is that the distance between the excitation center and the reflection center in one and the same cell amounts to precisely or approximately ±⅛ or ±⅜ of the wavelength. This wavelength corresponds to the quotient of the phase velocity of the surface wave divided by the frequency at which the transducer emits the maximal surface wave amplitude. Aside from wave excitation, reflection of waves is therefore a necessary condition for the existence of unidirectional properties. Excitation center and reflection center mean the excitation center and reflection center of the electrical potential of the surface acoustic wave. Natural unidirectionality is present if, despite the symmetrical structure of the transducer cell being considered, a greater wave amplitude is emitted in one direction—referred to as the forward direction—than in the opposite direction (backward direction). A symmetrical transducer consists, for example, of cells that consist of prongs having the same width, which have different polarity and whose prong centers have a distance of half a wavelength. If the distance of the excitation center from a reflection center of a cell does not deviate significantly from ±⅛ or ±⅜ of the wavelength, then the natural unidirectionality is referred to as being complete. Otherwise, it is referred to as being incomplete.
In the case of absence of a natural unidirectionality or natural unidirectional properties, the cells contain prongs having different widths, generally having different prong distances—in the case of more than two prongs per cell. Such a cell has an asymmetrical structure, considered in the propagation direction. If a transducer having a symmetrical structure possesses unidirectional properties, one refers to this property as natural unidirectionality.
In a special embodiment (P. V. Wright, “The natural single-phase unidirectional transducer: A new low-loss SAW transducer,” Proc. 1985 IEEE Ultrasonics Symposium, pages 58-63, hereinafter also referred to as [1]), the unidirectional properties of transducers on a quartz substrate, which are composed of cells having two fingers having a width of ¼ of the wavelength (i.e. having fingers of the same width) and a gap between them having a width of ¼ of the wavelength, are generated by rotating the propagation direction of the surface acoustic waves away from the crystalline X axis. During propagation of the surface acoustic waves in the direction of the X axis, there are no unidirectional properties for the transducer type used. More detailed information concerning a propagation direction of surface acoustic waves having natural unidirectionality can be found in another special embodiment (T. Thorvaldsson and B. P. Abbott, “Low loss SAW filters utilizing the natural single phase unidirectional transducer (NSPUDT),” Proc. 1990 IEEE Ultrasonics Symposium, pages 43-48, referred to as [2]). The crystal section of quartz and the propagation direction are determined by the Euler angles (0°, 124°, 25°.) This combination is characterized by complete natural unidirectionality. Another example for natural unidirectionality is langasite (La3Ga5SiO14) having the Euler angles (0°, 138.5°, 26.6°) (D. P. Morgan, S. Zhgoon, A. Shvetsov, E. Semenova, and V. Semenov, “One-port SAW resonators using Natural SPUDT substrates,” Proc. IEEE Ultrasonics Symposium, pages 446-449, referred to as [3]). In this case, the natural unidirectionality is incomplete.
A transducer having natural unidirectionality was also already proposed, in which the prongs are oriented perpendicular to a direction R. The direction R is directed parallel to a monad or triad axis of rotation of the substrate crystal, and the derivation dv/dθ=0 holds true for the direction R, wherein v is the phase velocity of the surface wave and θ is an angle deviation of the perpendicular to the prong direction from this direction R, The directions perpendicular to a mirror plane of the substrate crystal are excepted, and for the arrangement of the interdigital electrode structure, the crystal substrate surfaces parallel to a mirror plane of the substrate crystal or perpendicular to a dyad, tetrad, or hexad axis of rotation of the substrate crystal are excepted (DE 10 2010 028 007.0).
In another known special embodiment, transducers having natural unidirectionality with opposite forward directions are implemented on a LiNbO3 substrate having the Euler angles (0°, 38°, 20°) in that the electrode structure of one of the transducers consists of copper, and the electrode structure of the second transducer, in each instance, consists of aluminum (M. Takeuchi, “Natural single-phase unidirectional transducer on a LiNbO3 substrate for SAW sensor applications,” Proc. 2008 IEEE Frequency Control Symposium, pages 152-155).
The transducers described in this document, having natural unidirectionality, have the disadvantage that the technological effort in the production of components that contain these transducers is too great, as a result of the fact that the electrode structure of one of the transducers consists of copper and the electrode structure of the second transducer consists of aluminum, that copper and aluminum are not suitable for higher temperatures, and that copper is not suitable for those types of package that are carried out in an oxygen-rich atmosphere.